1. Field of the Invention
The present invention relates generally to apparatus and methods for metering of solid particulate materials, and more particularly, but not by way of limitation, to apparatus and methods for metering bulk cement to be mixed for cementing an oil or gas well.
2. Description of the Prior Art
During the cementing of an oil or gas well large volumes of cement must be mixed over a long interval of time while the cement is being pumped down into the well to cement a well casing into a bore hole. During this operation bulk dry cement is pneumatically conveyed from a bulk supply system through a cement metering device to a mixer such as a vertically oriented axial flow mixer.
The metering device previously used by the assignee of the present invention, Halliburton Services of Duncan, Okla., included a valve housing having a cylindrical valve bore with a rotary valve element disposed in the housing. The valve housing had a housing outlet located below the rotary valve element and had a lateral inlet opening. The rotatable valve element was designed such that as the valve element rotated from a closed position wherein the inlet opening was closed, the inlet opening would begin opening at the bottom of the opening and would increase the size of the bottom opening as the valve element was rotated toward a fully open position.
One difficulty which has been encountered with the abovedescribed device, and other available metering devices for bulk cement, is that during well cementing operations the metering device will tend to plug off during periods of low cement flow rates.
There is a need for an improved bulk cement metering device which can operate at relatively low flow rates without plugging off, and yet which retains the capability of handling the high flow rates of peak demand periods.